At a front end part of a communication terminal such as a mobile phone, use is made of a branching filter for branching transmission/reception frequencies.
A branching filter has an antenna terminal, transmitting terminal, and receiving terminal. A transmitting filter is arranged between the antenna terminal and the transmitting terminal, and a receiving filter is arranged between the antenna terminal and the receiving terminal. In the communication terminal, a transmitting circuit and a receiving circuit are arranged in a later stage of the branching filter. The branching filter has a function of branching the transmission signal from the transmitting circuit to the antenna terminal and branching the reception signal which was received at the antenna terminal to the receiving circuit.
In this type of a branching filter, in order to prevent the transmission signal from flowing to the receiving circuit or preventing the reception signal from flowing to the transmitting circuit, the transmitting filter and the receiving filter are matched.
However, depending on the design of the individual filters, even in a state where the best match is secured, for example, sometimes the impedance from the transmitting filter to the receiving filter becomes a bit lower than the impedance to the antenna and sometimes the impedance from the antenna to the transmitting filter becomes a bit lower than the impedance to the receiving filter. This being so, a signal which inherently should pass from the transmitting filter to the antenna flows to the receiving filter, a signal which should have been input from the antenna to the receiving filter flows to the transmitting filter, etc., so the isolation characteristics are degraded.
The transmitting filter and receiving filter which are used in the branching filter are configured by surface acoustic wave filters having IDT electrodes formed on piezoelectric substrates in many cases. In a conventional branching filter, the isolation characteristics have been improved by adjusting the impedances of the filters by changing the distance between electrode fingers of the IDT electrode or the number of electrode fingers or the like (see for example Patent Literature 1). In this regard, as the required specifications which are demanded from a branching filter, other than the isolation characteristics, attenuation characteristics become important as well. This is because, if either the isolation characteristics or attenuation characteristics are poor, the quality of communication of the mobile phone deteriorates or other problems occur.
With the conventional method of adjustment, however, the range of possible adjustment was limited. Therefore, it was difficult to satisfy both of the required specifications of the isolation characteristics and the attenuation characteristics.
The present invention was made in order to solve the above problems and provides a branching filter capable of improving the isolation characteristics while suppressing degradation of the attenuation characteristics and provides a communication module component using the same.